This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No. 2001-32244 filed in Japan on Feb. 8, 2001, which is herein incorporated by reference.
The present invention relates to a steel workpiece oil quenching method and, in particular, to a method for processing a steel workpiece in a marquenching manner.
In general, when a steel workpiece maintained at a quenching temperature is immersed in quenching oil, the steel workpiece is cooled through the three stages of a vapor film stage (high-temperature region), a boiling stage (intermediate-temperature region) and a convection stage (low-temperature region). It is known that a cooling speed in the vapor film stage is slow and a cooling speed in the boiling stage is three to ten times faster than the above-mentioned speed. A high-temperature quenching oil (hot quenching oil), of which the cooling speed in the intermediate-temperature and low-temperature regions is slower than that of a low-temperature quenching oil (cold quenching oil), is therefore able to reduce the distortion attributed to a quenching transformation. However, it is also known that a thermal distortion attributed to a temperature difference in the high-temperature region tends to easily occur since the time of the vapor film stage is short and the end temperature in the vapor film stage is high. If the quenching oil is put in a reduced pressure state, as shown in FIG. 2, the time of the vapor film stage is prolonged by the reduction of the boiling point, and the end temperature in the vapor film stage is lowered. Accordingly, as a method for reducing the deformation attributed to the quenching taking advantage of the above-mentioned phenomenon, there has been put in practice a method for performing quenching by immersing a steel workpiece maintained at a quenching temperature in a high-temperature quenching oil or a method for performing quenching by immersing the steel workpiece in a quenching oil under a reduced pressure.
On the other hand, as an oil quenching method of a steel workpiece such as a gear, there is a method (marquenching method) for rapidly cooling a steel workpiece maintained at a specified quenching temperature to a temperature slightly higher than the martensite transformation start point (Ms point) by immersing the steel workpiece in a high-temperature coolant at a temperature slightly higher than the martensite transformation start point (Ms point), thereafter cooling the steel workpiece in the atmospheric air by taking out the steel workpiece out of the high-temperature coolant at a point of time when the entire steel workpiece comes to have roughly same temperature, and thereby effecting the martensite transformation. This method, which can reduce the quenching distortion and the quenching variation, has the problem that the cooling speed causes a temperature difference between the placement positions of workpieces in a tray and between the portions of a workpiece due to the cooling in the atmospheric air, and consequently the quenching distortion and the quenching variation attributed to the temperature difference cannot be avoided.
As a method for solving this problem, there has been proposed a method for rapidly cooling a steel workpiece maintained at a specified temperature by immersing the steel workpiece in a high-temperature coolant at a temperature higher than the martensite transformation start temperature and thereafter immersing the steel workpiece in a low-temperature coolant at a temperature lower than the martensite transformation start temperature at the point of time when the entire steel workpiece comes to have roughly same temperature (as described in Japanese Patent Laid-Open Publication No. 2-101113), a method for providing a circulation system for circulating a quenching oil, using a quenching bath that has a hood for surrounding the workpiece, immersing the workpiece inside the hood in a state in which the circulation system is stopped, raising the temperature of the quenching oil inside the hood close to the martensite transformation start temperature (Ms point) by the heat of the workpiece, and then rapidly cooling the workpiece to a temperature lower than the martensite transformation start temperature by circulating the quenching oil in the circulation system at the point of time when the entire workpiece has roughly same temperature (as described in Japanese Patent Laid-Open Publication No. 6-279838) or the like.
However, from the viewpoint of the construction and structure of the quenching apparatus, the former method, which needs not only the high-temperature coolant and the low-temperature coolant but also the quenching bath for the high-temperature coolant and the quenching bath for the low-temperature coolant, has the problem that the quenching apparatus becomes inevitably increased in size and complicated and has poor maintenance. The latter method, which solves the problem of the former method, has the problem that the quenching bath itself is complicated. Moreover, from the viewpoint of quenching distortion and variation, both are the systems for putting the entire steel workpiece into roughly same temperature by immersing the steel workpiece in the coolant, and therefore, it is difficult to bring the coolant that serves as a thermal medium in contact with all workpieces on the tray or the entire portions of the workpiece uniformly and sufficiently, and consequently a temperature difference occurs between the steel workpieces or the portions of the steel workpiece in the soaking stage. Accordingly, although those methods have the effect of reducing the quenching distortion and variation, the effect are not satisfactory.
As a result of detailed researches for solving the aforementioned problems, it was discovered that these quenching distortion and variation were attributed to the temperature difference between the steel workpieces or the portions of the steel workpiece during the martensite transformation and to the fact that the cooling speed in the high-temperature region (not lower than about 550xc2x0 C.) was too fast.
Accordingly, the present invention has the object of reducing the temperature difference between steel workpieces or the portions of a steel workpiece in the martensite transformation stage and making the cooling speed in a high-temperature region (not lower than about 550xc2x0 C.) a slow speed sufficient for restraining the thermal distortion, thereby reducing the quenching distortion and quenching variation.
In order to achieve the aforementioned object, a steel workpiece oil quenching method of the present invention comprises the steps of: rapidly cooling a steel workpiece maintained at a specified quenching temperature by immersing the steel workpiece in a high-temperature quenching oil until a temperature of a specified portion of the steel workpiece reaches a temperature just above a martensite transformation start point (Ms point); thereafter taking the steel workpiece out of the high-temperature quenching oil to soak the steel workpiece by heat possessed by the steel workpiece; and cooling the steel workpiece by subsequently immersing the steel workpiece in the high-temperature quenching oil.
According to one embodiment of the present invention, the steel workpiece is rapidly cooled in a rapid cooling process until a temperature of its portion producing the largest deformation amount (in the concrete, a portion having a small internal volume with respect to a unit area of the steel workpiece (for example, a gear tooth, a corner portion of a prismatic workpiece, or the like, hereinafter referred to as xe2x80x9ca sharp portionxe2x80x9d)) reaches a temperature just above the Ms point of the portion.
The present invention is based on the following knowledge. That is, according to the method of the present invention, the steel workpiece is first cooled to a temperature just above the martensite transformation start point (Ms point) by being immersed in the high-temperature quenching oil. In this case, it is proper to take the steel workpiece out of the high-temperature quenching oil at the point of time when the entire steel workpiece is cooled to a temperature just above the aforementioned Ms point. However, the practical steel workpiece is generally subjected to a carburizing process immediately before being quenched. Therefore, the concentration of carbon dispersed inside the steel workpiece is not uniform throughout the entire steel workpiece, and the Ms point is sometimes varied with portions. For example, in a sharp portion of a steel workpiece, the carbon concentration in its surface portion becomes higher than that of a portion (for example, a portion that is not carburized inside the steel workpiece, i.e., a non-carburized portion) other than the sharp portion. Therefore, the Ms point (Ms2) of the surface portion of the sharp portion becomes lower than the Ms point (Ms1) of the non-carburized portion, and there consequently occurs a variation in the amount of deformation. However, the portion that belongs to the steel workpiece and is other than the sharp portion, i.e., the non-carburized portion generally has little deformation even when being quenched. Even if the temperature of the non-carburized portion becomes just below the Ms point (Ms1) of the portion, the characteristics of the steel workpiece receive little bad influence. When the temperature of the sharp portion producing the largest deformation amount is just above the Ms point (Ms2) of the portion, the variation in the amount of deformation of the entire steel workpiece can be reduced by soaking. The present invention is based on such knowledge.
For the aforementioned quenching oil, of which the type, temperature and quantity are set according to the workpiece, there is normally adopted a high-temperature quenching oil corresponding to No. 1 or 2 of Type 2 of JIS K2242. Moreover, the temperature of the quenching oil is set within a range of 100 to 170xc2x0 C. It is proper to set the quantity of the quenching oil to a value at which the aforementioned setting temperature is not largely varied by primary quenching.
Moreover, after the rapid cooling, the steel workpiece is retained in an upper space located above the quenching oil inside the oil quenching chamber in order to thermally uniform or soak the workpiece. The atmospheric temperature of the upper space is normally roughly equal to the temperature of the high-temperature quenching oil. It is difficult to uniquely determine the time during which the workpiece obtained through the primary quenching is soaked by the heat possessed by the workpiece itself in the upper space of the oil quenching chamber until the workpiece comes to have a temperature just above the martensite transformation start point (Ms point) because the time is varied depending on the size, material and so on of the workpiece. The time is normally set to 30 to 300 seconds.
A temperature difference ascribed to the placement position of the workpiece in the tray and a temperature difference between the portions of a workpiece can further be reduced by the so-called marquenching process for immersing again the workpiece in the high-temperature quenching oil after the soaking.
According to one embodiment of the present invention, the internal pressure in the oil quenching chamber during the quenching is set to 7 to 75 KPa or, more preferably, to 8 to 40 KPa. This is because the vapor film stage in a vacuum higher than 7 KPa becomes too long to obtain a sufficient quenching hardness and a sufficient depressurizing effect cannot be obtained, failing in restraining the thermal distortion, in a vacuum lower than 75 KPa.